Samir Salim

Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog

We complete construction of a catalog containing improved astrometry and new optical/infrared photometry for the vast majority of NLTT stars lying in the overlap of regions covered by POSS I and by the second incremental Two Micron All Sky Survey (2MASS) release, approximately 44% of the sky. The epoch 2000 positions are typically accurate to 130 mas, the proper motions to 5.5 mas yr-1, and the V-J colors to 0.25 mag. Relative proper motions of binary components are measured to 3 mas yr-1. The false-identification rate is ~1% for 11 V 18 and substantially less at brighter magnitudes. These improvements permit the construction of a reduced proper-motion diagram that, for the first time, allows one to classify NLTT stars into main-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We in turn use this diagram to analyze the properties of both our catalog and the NLTT catalog on which it is based. In sharp contrast to popular belief, we find that NLTT incompleteness in the plane is almost completely concentrated in MS stars, and that SDs and WDs are detected almost uniformly over the sky δ > -33°. Our catalog will therefore provide a powerful tool to probe these populations statistically, as well as to reliably identify individual SDs and WDs.

M Dwarfs from Hubble Space Telescope Star Counts. IV.

We study a sample of about 1400 disk M dwarfs that are found in 148 fields observed with the Wide Field Camera 2 (WFC2) on the Hubble Space Telescope and 162 fields observed with pre-repair Planetary Camera 1 (PC1), of which 95 of the WFC2 fields are newly analyzed. The method of maximum likelihood is applied to derive the luminosity function and the Galactic disk parameters. At first, we use a local color-magnitude relation and a locally determined mass-luminosity relation in our analysis. The results are consistent with those of previous work but with considerably reduced statistical errors. These small statistical errors motivate us to investigate the systematic uncertainties. Considering the metallicity gradient above the Galactic plane, we introduce a modified color-magnitude relation that is a function of Galactic height. The resultant M dwarf luminosity function has a shape similar to that derived using the local color-magnitude relation but with a higher peak value. The peak occurs at MV ~ 12, and the luminosity function drops sharply toward MV ~ 14. We then apply a height-dependent mass-luminosity function interpolated from theoretical models with different metallicities to calculate the mass function. Unlike the mass function obtained using local relations, which has a power-law index α = 0.47, the one derived from the height-dependent relations tends to be flat (α = -0.10). The resultant local surface density of disk M dwarfs (12.2 ± 1.6 M☉ pc-2) is somewhat smaller than the one obtained using local relations (14.3 ± 1.3 M☉ pc-2). Our measurement favors a short disk scale length, H = 2.75 ± 0.16 (statistical) ± 0.25 (systematic) kpc.

Improved Astrometry and Photometry for the Luyten Catalog. I. Bright Stars

We outline the construction of a refined version of the New Luyten Two-Tenths (NLTT) catalog of high proper motion stars, which will contain improved astrometry and photometry for the vast majority of the ~59,000 stars in NLTT. The bright end is constructed by matching NLTT stars to Hipparcos, Tycho-2, and Starnet; the faint end by matching to USNO-A and the Two Micron All Sky Survey (2MASS). In this first paper, we detail the bright-end matching procedure. We show that for the majority of stars in his catalog, Luyten measured positions accurate to 1 even though he recorded his results much more coarsely. However, there is a long tail of position errors, with one error as large as 11?. Proper-motion errors for the stars with small position errors are 24 mas yr-1 (1 ?) but deteriorate to 34 mas yr-1 for stars with inferior positions. NLTT is virtually 100% complete for V 11.5 and |b| > 15?, but completeness in this magnitude range falls to ~75% at the Galactic plane. Incompleteness near the plane is not uniform, but is rather concentrated in the interval -80? < l < 20?, where the Milky Way is brightest.

Classifying Luyten Stars Using an Optical-Infrared Reduced Proper-Motion Diagram

We present a V-J reduced proper-motion (RPM) diagram for stars in the new Luyten two-tenths (NLTT) catalog. In sharp contrast to the RPM diagram based on the original NLTT data, this optical-infrared RPM diagram shows distinct tracks for white dwarfs, subdwarfs, and main-sequence stars. It thereby permits the identification of white dwarf and subdwarf candidates that have a high probability of being genuine.

Photometric Microlens Parallaxes with the Space Interferometry Mission

Astrometric measurements of microlensing events can in principle determine both the parallax E and the proper motion μ of an individual event, which (combined with the Einstein timescale tE) in turn yield the mass, distance, and transverse velocity of the lens. We show, however, that the parallax measurements are generically several orders of magnitude less precise than the proper-motion measurements. Fortunately, astrometric measurements by the Space Interferometry Mission (SIM) are simultaneously photometric measurements, and since SIM will be in solar orbit, these allow SIM to be used as a classical (photometric) parallax satellite. We show that SIM photometric parallaxes are of precision comparable to that of its astrometric proper-motion measurements. For I = 15 bulge stars, complete solutions with ~5% accuracy in mass, distance, and transverse velocity can be obtained from about 5 hr of observation, 100-10,000 times shorter than would be required for a purely astrometric solution of similar precision. Thus, it should be possible to measure directly the mass functions of both the bulge and the inner disk (including both dark and luminous objects) with only a few hundred hours of SIM observations.

Nearby Microlensing Events: Identification of the Candidates for theSpace Interferometry Mission

The Space Interferometry Mission (SIM) is the instrument of choice when it comes to observing astrometric microlensing events where nearby, usually high proper motion, stars (lenses) pass in front of more distant stars (sources). Each such encounter produces a deflection in the sources apparent position that, when observed by SIM, can lead to a precise mass determination of the nearby lens star. We search for lens-source encounters during the 2005-2015 period using Hipparcos, ACT, and NLTT to select lenses, and USNO-A2.0 to search for the corresponding sources, and rank these by the SIM time required for a 1% mass measurement. For Hipparcos and ACT lenses, the lens distance and lens-source impact parameter are precisely determined so that the events are well characterized. We present 32 candidates beginning with a 61 Cyg A event in 2012 that requires only a few minutes of SIM time. Proxima Centauri and Barnards star each generate several events. For NLTT lenses, the distance is known only to a factor of 3, and the impact parameter only to 1. Together, these produce uncertainties of a factor ~10 in the amount of SIM time required. We present a list of 146 NLTT candidates and show how single-epoch CCD photometry of the candidates could reduce the uncertainty in SIM time to a factor of ~1.5.

Sagittarius A* “Visual Binaries”: A Direct Measurement of the Galactocentric Distance

We present a new geometrical method for measuring the distance to the Galactic center (R0) by solving for the Keplerian orbit of individual stars bound to the black hole associated with Sgr A* from radial velocity and proper-motion measurements. We identify three stars to which the method may be applied, and show that two of them individually could produce 1%-5% accuracy of R0 after 15 years of observing, and 0.5%-2% after 30 years of observing, depending on what the orbital parameters of these stars turn out to be. Further, we show that combining the measurements of the three stars, together with the constraints that they must orbit a common center having a common mass, produces considerable improvement in the R0 determination over simple averaging of the three determinations. Using a power-law stellar density model, we find the probability distribution over allowed orbital parameter space. Then we perform a Monte Carlo simulation to pick orbits and find the corresponding R0 uncertainty. We find that there is a 40% chance of getting a 5% measurement of R0 as early as the year 2003. Seven years later, 3% precision is almost guaranteed, while there is a 30% chance of obtaining 1% precision. All these estimates assume that annual position measurements will continue to be made with the σρ = 2 mas precision recently reported by Ghez and coworkers. The precision of the distance measurement is relatively insensitive to the radial velocity errors, provided that the latter are less than 50 km s-1. Another useful result that would come along with R0 is the heliocentric radial velocity of Sgr A*. Besides giving an estimate of R0 that is better than any currently in use, a great advantage of this method is that it is free from the kind of systematic errors that affect other methods currently in use. Moreover, a sustained observing campaign will produce predictable improvements in the measurement of R0 whether the quality of the position and velocity measurements improves or not.

A search for nearby counterparts to the moving objects in the Hubble Deep Field

Ibata et al. have recently discovered very faint, moving objects in the Hubble Deep Field (HDF). The number, apparent magnitudes and proper motions of these objects are consistent with old white dwarfs making up part of the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark-halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the HDF. We conclude that it is unlikely that hydrogen-atmosphere white dwarfs make up a significant fraction of the Galactic dark matter. No limits can be placed as yet on helium-atmosphere dwarfs from optical searches.

Discovery of the Bright Trans-Neptunian Object 2000 EB173

We describe the discovery circumstances and photometric properties of 2000 EB173, now one of the brightest trans-Neptunian objects (TNOs) with opposition magnitude mR =1 8:9 and also one of the largest Plutinos, found with the drift-scanning camera of the QUEST Collaboration, attached to the 1-m Schmidt telescope of the National Observatory of Venezuela. We measure B V =0 :99 0:14 and V R =0 :57 0:05, a red color observed for many fainter TNOs. At our magnitude limitmR =2 0:1 0:20, our single detection reveals a sky density of 0.015 (+0.034, -0.012) TNOs per deg 2 (the error bars are 68% condence limits), consistent with fainter surveys showing a cumulative number proportional to 10 0:5mR. Assuming an inclination distribution of TNOs with FWHM exceeding 30 deg, it is likely that one hundred to several hundred objects brighter than mR =2 0:1 remain to be discovered.

Searching for Failed Supernovae with Astrometric Binaries

Stars in the mass range 8 MdMd30 Mare thought to end their lives as luminous supernovae that leave behind a neutron star. However, if a substantial fraction of these stars instead ended as black hole rem- nants, without producing a supernova (a failed supernova), how would one know? We show that, under plausible assumptions, the Hipparcos catalog should contain � 30ffail astrometric binaries with black hole companions, where ffail is the fraction of supernovae that fail. Since no black hole astrometric binaries are found in Hipparcos, one might like to conclude that such failed supernovae are very rare. However, the most important assumption required for this argument, the initial companion mass function (ICMF) of G stars (the majority of Hipparcos stars) in the high-mass companion regime, is without any observational basis. We show how the ICMF of G stars can be measured using future space-based astrometric missions, thereby per- mitting an accurate measurement of the rate of supernovae that fail. Subject headings: astrometry — binaries: general — stars: neutron — supernovae: general On-line material: color figures